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****** Menthol from
delta-3-Carene
******

(+)-delta-3-Carenc from Western U.S. turpentine provides
another monoterpeiie raw material for the potential
production of (-)-menthol. Catalytic isomerization provides
the needed (+)-delta-2-carene required for either of two
alternate synthetic routes. In one approach, reaction of
(+)-2-carelie with peracid yields
(+)-cis-2,8-p-menthadien-l-ol via the intermediate
cis-2-carene epoxide. Allylic displacements with acetic or
formic acid in a buffered solution affords a mixture of cis-
and trans-piperitenyl acetates (or formates) which on
hydrolysis give (+)-cis- and (-)-trans-piperitenol. These
isomeric alcohols are separated by fractional distillation
and the cis-isomer is recycled by the same buffered
carboxylic acid system used above to mixed cis & trans
piperitenyl esters. In this manner pure
(-)-trans-piperitenol is obtained with minimum losses due to
(+)-cis isomer formation. The
(-)-trans-piperitenol is desired here so as to be
able to selectively obtain (-)-menthol directly on
hydrogenation. Hydrogenation over Pd/C affords better thin
70% (-)-menthol along with some undesired (-)-isomenthol.
Efficient fractional distillation gives (-)-menthol of good
purity. Unfortunately, the (-)-isomenthol obtained in the
last step of this synthesis is of the opposite optical
series, but since this can be catalytically converted to
racemic menthol under strong catalytic conditions this
by-product is still salable.

A second route, developed by Hercules, Inc., involves
pyrolysis of (+)-delta-2-carene to
(+)trans-2,8-p-menthadiene with generation of an asymmetric
center at C-1 which is carried through the remainder of the
synthesis. Isomerization of this 2,8-menthadiene to
(+)-2,4(8)-p-menthadiene can be accomplished either
catalytically in the presence of strong bases (e.g.,
potassium t-butoxide) or via
hydrochlorination-dehydrochlorination. Treatment of
(+)-2,4(8)-p-menthadiene with hydrogen chloride affords
8-chloro-3-p-menthene which can be reacted with sodium
acetate and acetic acid to give mixed (cis/trans) pulegol
esters via allylic displacements. Hydrolysis affords (-)-cis
and (+)-trans-pulegol. Because the absolute configuration of
C-1 is fixed in this system, reduction of either pulegol
isomer provides menthol isomers which can be readily
equilibrated to predominently (-)-menthol. More
specifically, reduction of (-)-cis-pulegol affords
(-)-menthol and (+)-neoisomentliol, while (+)-trans-pulegol
gives (+)-isomentliol and (+)-neomenthol. Purification of
equilibrated menthol isomers is carried out by fractional
distillation and crystallization of menthol derivatives.
This latter process is intrinsically one of the most
attractive potential routes to (-)-menthol developed to date
but was never commercialized.